It has been shown previously that it is important to supply a static negative pressure (or head) at the orifices of an ink jet to enhance print quality. By doing so, a negative meniscus draws any ink at the orifices back into the pen, and provides a cleaner, more uniform ejection surface.
In a portable or disposable pen, the importance of a negative head is even more important, because the ink must be contained even in transit, at any altitude, and unde shock and vibration. In the case of a portable disposable pen, the only mechanisms holding the ink into the pen when the orifices are face down in the vertical direction are surface energy related.
As shown in FIG. 1A, the pressure P1 exerted on the liquid 10 in the reservoir 20 by the orifice 30 is related to the radius of curvature, r1, and the surface energy of the fluid .gamma.. Thus P1=2.gamma./rl. The pressure Pa exerted by the fluid due to an external acceleration such as gravity or external shock is related to the fluid density .pi., head height h, and acceleration a. Thus Pa=.pi.ah. If the orifice diameter D is small enough, an equilibrium condition will be achieved such that ink will not flow from the orifices. If the orifice plate wets well in this attitude, the contact angle .phi.1 of the fluid, on the orifice surface will be insufficient to exert sufficient pressure P2 to sustain Pa as shown in FIG. 1B. Thus P2=2.gamma./r2&lt;P1.
The prior art suggests that an antiwett coating should be applied around the orifice, to increase the contact angle .phi.2, as shown in FIG. 1C, thus increasing the capillary pressure. In practice this approach has two major drawbacks. First, due to a sudden shock (increased a), a blob of ink will emerge which may have sufficient radius r to overcome the equilibrium condition. Second, and more importantly, most antiwet compounds are attacked by the dye in the ink since an important quality of a dye is that it chemically bonds itself to a surface. This poisons the antiwet coating and drops the contact angle back to a low value.
Another way to contain the ink in the reservoir includes valves, which however are large, clumsy and expensive.